1. Field of the Invention
The present invention relates to a jet-propulsion personal watercraft (PWC) which ejects water rearward and planes on a water surface as the resulting reaction, and more particularly to an exhaust system (device) of the personal watercraft.
2. Description of the Related Art
In recent years, so-called jet-propulsion personal watercraft have been widely used in leisure, sport, rescue activities, and the like. The personal watercraft is configured to have a water jet pump that pressurizes and accelerates water sucked from a water intake generally provided on a hull bottom surface and ejects it rearward from an outlet port. Thereby, the personal watercraft is propelled.
In the jet-propulsion personal watercraft, a steering nozzle provided behind the outlet port of the water jet pump is swung either to the right or to the left, to change the ejection direction of the water to the right or to the left, thereby turning the watercraft to the right or to the left.
In the personal watercraft, a straddle-type seat is placed to span between substantially the center portion and its rear portion in the longitudinal direction of the watercraft, and an engine is contained in an engine room located beneath the seat and surrounded by a hull and a deck. A crankshaft of the engine projects rearwardly in the longitudinal direction and its rear end is coupled to a pump shaft of the water pump, thereby driving the water jet pump.
In an exhaust system of the personal watercraft, an exhaust manifold is continuously connected to an exhaust port of the engine, and its downstream end is connected to an upstream end of an exhaust pipe in an exhaust gas flow path to allow an exhaust gas from the engine to be discharged from the downstream end of the exhaust pipe outside the watercraft and a muffler is provided in the exhaust pipe.
In general, the downstream end of the exhaust pipe projects over water rearwardly from a transom board of the watercraft, and the exhaust gas is discharged from the downstream end of the exhaust pipe outside the watercraft.
Another exhaust system is disclosed in Japanese Laid-Open Patent Application Publication No. Sho. 60-157994, which was filed by the applicant of the present invention. In this system, a rotational shaft of an impeller is a hollow shaft and a hollow portion of this hollow shaft is connected to the downstream end of the exhaust pipe, thereby discharging the exhaust gas into water. In a further exhaust system similar to this system, which is disclosed in Japanese Laid-Open Patent Application Publication No. Sho. 54-151293, a propeller shaft of the outboard engine has a hollow structure and the downstream end of the exhaust pipe of the engine is connected to the propeller shaft, thereby discharging the exhaust gas into water.
However, in these prior art systems, the downstream end of the exhaust pipe is connected to the impeller shaft or the propeller shaft rotated by the engine. Therefore, a seal structure of the portion where the exhaust passage formed in the rotational shaft is connected to the downstream end of the fixed exhaust pipe is complex. Consequently, there is some limitation to greater seal durability.
For the purpose of a simplified seal structure, Japanese Utility Model Publication No. Sho. 50-99691 discloses that an exhaust outlet pipe connected to an exhaust pipe is arranged so as to traverse across a water flow passage of the pump to the center of an impeller at the rear end of the impeller, and from there, extends rearward along the water flow passage to a pump nozzle, thereby allowing the exhaust gas to be discharged into the pump nozzle.
Despite the capability of decreasing the exhaust noise, this constitution requires high seal performance, since the exhaust pipe is connected to the water flow portion of the water jet pump. This results in a complex structure.
Also, since the personal watercraft is commonly used in sport activities, it is sometimes inverted or almost inverted. When the personal watercraft is inverted, in the above exhaust system in which the downstream end of the exhaust pipe projects rearwardly from the overwater portion of the transom board, the downstream end of the exhaust pipe is submerged in water and thereby the water enters the engine from the exhaust pipe. Accordingly, against such inversion, a bellows valve is generally provided at the downstream end of the exhaust pipe, or otherwise the muffler has an internal labyrinth structure, for the purpose of preventing water ingress into the vicinity of the exhaust port of the engine. However, the provision of the bellows valve at the downstream end of the exhaust pipe or the complex labyrinth structure in the muffler causes an increase in exhaust resistance (back pressure in the exhaust passage), which would lead to a reduced engine power.
The present invention addresses the above-described condition, and an object of the present invention is to provide a personal watercraft comprising a simple and durable exhaust system capable of reducing an exhaust noise without degrading a function of a water jet pump. Another object of the present invention is to provide a personal watercraft comprising an exhaust system capable of preventing water ingress into an engine even when the watercraft is inverted.
In accordance with a first aspect of the present invention, there is provided a personal watercraft comprising: a water jet pump including an outlet port, the water jet pump pressurizing and accelerating water taken in from outside of the watercraft and ejecting the water from the outlet port to propel the watercraft as a reaction of the ejecting water; a pump room that contains the water jet pump and communicates with an ambient side at a rear side of the pump room; an exhaust chamber defined by double walls formed by double-walling an outer peripheral portion of at least part of a pump casing of the water jet pump; an exhaust inlet provided in an outer wall of the double walls defining the exhaust chamber and connected to a downstream end of an exhaust pipe extending from the engine in an exhaust gas flow path; and an exhaust outlet provided in an outer wall of a portion of the double walls which is exposed in the pump room and opened in the pump room.
In the personal watercraft so constituted, the exhaust gas from the engine flows through the exhaust inlet in the outer wall of the double walls formed in the outer peripheral portion of the pump casing of the water jet pump and then through the inside of the exhaust chamber defined by the double walls, and is discharged from the exhaust outlet in the outer wall into the pump room communicating with the ambient side at its rear end. When the exhaust gas is discharged from the exhaust outlet into the pump room communicating with the ambient side, the pump room functions as the expansion chamber to reduce the exhaust noise of the exhaust gas. In addition, the exhaust noise of the exhaust gas being discharged from the rear side of the pump room outside the watercraft is absorbed by sprays being raised in the vicinity of the ejection nozzle of the water jet pump and minimized.
The exhaust system is attained merely by connecting the downstream end (rear flow end) of the exhaust line to the exhaust inlet in the double walls of the pump casing of the water jet pump for allowing the exhaust gas to be discharged outside without specially sealing a movable mechanism element. That is, the exhaust system is attained in a simple manner, and high durability and reliability is obtained in the sealed portion.
While it is necessary to form the double walls defining the exhaust chamber in the pump casing of the water jet pump, the double walls can be formed at a low cost because the pump casing is generally manufactured by casting. Or otherwise, the double walls may be formed by sheet metal construction at a low cost.
In the personal watercraft, the exhaust chamber may be formed in an upper portion of the pump casing of the water jet pump. With such constitution, since the exhaust gas is discharged outside the watercraft while planing on the water surface, the back pressure in the exhaust system is not increased, in contrast to the discharge into water. This follows that losses in the engine power caused by the back pressure are not generated.
In the personal watercraft, the exhaust chamber may be formed in a side portion of the pump casing of the water jet pump. Such constitution provides nearly the same functions as those given by forming the exhaust chamber in the upper portion of the pump casing.
In the personal watercraft, the exhaust outlet in the exhaust chamber may be opened in the direction orthogonal to the direction along which water flows inside the water jet pump. Even when the waves following behind the watercraft collide with the double-walled portion during cruising, such following waves enter the exhaust chamber through the exhaust outlets without keeping high energy.
In the personal watercraft, the exhaust inlet may be formed adjacent a front wall of the pump room and an opening is formed in the front wall to allow the exhaust inlet to be connected to a downstream end of the exhaust pipe therethrough. Such constitution increases rigidity of the connected portion.
In accordance with a second aspect of the present invention, there is further provided a personal watercraft comprising: a water jet pump including an outlet port, the water jet pump pressurizing and accelerating water taken in from outside of the watercraft and ejecting the water from the outlet port to propel the watercraft as a reaction of the ejecting water; an engine disposed in a body surrounded by a hull and a deck of the watercraft, for driving the water jet pump; and an exhaust line configured such that an upstream end of the exhaust line in an exhaust gas flow path is connected to an exhaust port of the engine, a downstream end of the exhaust line in the exhaust gas flow path is opened to the outside of the watercraft, and at least one part of a portion between the upstream end and the downstream end is located higher than a waterline of the watercraft when the watercraft is inverted.
In the personal watercraft so constituted, since at least one part of an intermediate portion of the exhaust line is located higher than the waterline of the inverted watercraft, the water ingress from the downstream end of the exhaust line into the engine is prevented by such higher portion.
Preferably, one of an exhaust inlet and an exhaust outlet of a muffler provided in the exhaust line may be located higher than the waterline of the inverted watercraft. The water ingress into the engine can be more effectively prevented because of a large capacity of the muffler.
Preferably, at least one part of a portion extending along an exhaust line from the muffler to the downstream end in the exhaust gas flow path is located higher than the waterline of the inverted watercraft.
Preferably, a portion of the exhaust pipe which is adjacent to a downstream end of the muffler may be located higher than the waterline of the inverted watercraft. Such constitution can prevent water ingress into the engine at the portion downstream (on the rear flow side) of the muffler.
In the personal watercraft, the downstream end of the exhaust line may be opened in a transom board of the watercraft, for discharging the exhaust gas outside the watercraft. This achieves a simple structure.
In the personal watercraft, the downstream end of the exhaust line may communicate with an exhaust chamber formed in part of a pump casing of the water jet pump which is exposed in a pump room communicating with the outside of the watercraft at a rear side of the pump room, the exhaust chamber being defined by a double-walled portion having an exhaust inlet and an exhaust outlet, and the exhaust gas flows through the inside of the exhaust chamber and is discharged outside the watercraft through the exhaust outlet and the pump room. With such constitution, the water ingress from the downstream end of the exhaust line into the engine is prevented, when the watercraft is inverted, and further, the exhaust noise of the exhaust gas is effectively reduced.
In the personal watercraft, the engine may be a four-cycle engine. In the four-cycle engine, the water ingress into the exhaust port of the engine can be also prevented.
In the personal watercraft, the exhaust outlet may be opened in the direction orthogonal to the direction along which water flows inside the water jet pump. Even when the waves following behind the watercraft collide with the double-walled portion during cruising, such following waves enter the exhaust outlets without keeping high energy.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.